Vinylidene fluoride (VdF) elastomers comprising various components have been proposed so far. Examples of such a known VdF elastomer are a copolymer of VdF and hexafluoropropylene (HFP) (VdF/HFP=60 to 15/40 to 85 in % by weight) (for example, JP-B-33-7394); a terpolymer of tetrafluoroethylene (TFE), VdF and HFP in which an amount of TFE is from 3 to 35% by weight and a weight ratio of VdF/HFP is from 2.33/1 to 0.667/1 (for example, JP-B-36-3495); a terpolymer of tetrafluoroethylene (TFE), VdF and HFP in which an amount of TFE is from 10 to 30% by weight and a weight ratio of VdF/HFP is from 1.6/1.0 to 4.0/1.0 (for example, JP-B-48-18957); a terpolymer comprising from 57 to 61% by weight of VdF, from 27 to 31% by weight of HFP and from 10 to 14% by weight of TFE (for example, JP-A-53-149291); and the like. Further there was proposed a process for preparing, in the presence of a chain transfer agent, a VdF elastomer comprising VdF and at least one other fluorinated ethylenically to unsaturated monomer having carbon atoms and at least the same number of fluorine atoms (JP-A-47-5944).
However the elastomer of the present invention explained hereinbelow has not been obtained by any of those means. This is because those patent publications take uniformity of a polymer structure into consideration and are silent with respect to setting of reaction conditions directed to a reaction to produce branch chains and thus even concrete means to solve problems are not suggested.
Fluorine-containing elastomers are molded by means of usual rubber processing equipment. Namely, the elastomers are processed in the order of kneading with a kneading roll or kneader, molding with an extruder, calendar roll or press, primary vulcanizing by press injection and then finally secondary vulcanizing with an oven.
However only with the above-mentioned fluorine-containing elastomers, there are problems with mold-processing, namely processability with a roll at kneading and mold-releasing property at compression molding are inferior, contamination of a metal die cannot be fully prevented and flowability at injection molding is not good. Therefore in order to solve those problems with mold-processing, widening of a molecular weight distribution has been proposed (for example, JP-A-52-62391, JP-A-4-209643, etc.).
On the contrary, there arose a tendency that by increasing a content of low molecular weight components, resistance against compression set was lowered, and mechanical strength such as elongation and tensile strength and further solvent resistance were lowered.
As mentioned above, physical properties such as resistance against compression set and mechanical properties are considered contrary to mold-processability. Actually in order to take a balance of physical properties and mold-processability, a molecular weight and a molecular weight distribution of a polymer are adjusted, a structure of the polymer end is changed or any of properties are sacrificed depending on applications.
Further with respect to the fluorine-containing elastomer composition for a sealing material such as O-ring and gasket, a particularly low compression set and further an efficient vulcanization (high vulcanization rate and high crosslinking density) are required. Compression set is an important factor for evaluating a sealing performance, and if the compression set is high, sealing performance is lost in a short period of time. Vulcanizability is a factor greatly influencing productivity in mold-processing, and it is desired that the vulcanizability is as high as possible within the range satisfying required physical properties. Also particularly in injection molding, an excellent flowability is required. If the flowability is inferior, pouring into a metal die is difficult and a molding material must be used in an amount more than required. Also there is a case where a thickness of a molded article becomes non-uniform.
In order to make vulcanization efficient, there is proposed that the number of ionic ends (or acid ends) attributable to ammonium persulfate which has been used as a polymerization initiator is minimized as low as possible. For example, there are a method of using an oil soluble organic peroxide as a polymerization initiator (for example, JP-A-6-302487, JP-A-8-301940) and a method of using a fluoroalkyl sulfinate and organic peroxide together (U.S. Pat. No. 5,256,745). However in those methods, processability other than vulcanizability cannot be satisfied.
To enhance the both of vulcanization efficiency and resistance against compression set simultaneously, there is proposed a method of adding a phosphonate compound or the like as a vulcanization accelerator (JP-A-62-54750). However even in this method, there remains a problem that enhancement of processability cannot be attained.
Further though the flowability is intended to be improved by decreasing a molecular weight as mentioned above, resistance against compression set and vulcanization efficiency are still insufficient.
JP-A-10-130447 discloses a technique that by vulcanizing in an oven for 0 to 30 minutes, a compression set becomes not more than 20%, a thermal stability is high and a content of a low molecular weight component having a molecular weight of not more than 10,000 is small. However according to concrete preparation examples of the publication, even if the vulcanization in an oven is carried out for not less than 30 minutes, compression set does not change as compared with that of 30-minute vulcanization in an oven and is only about 14%. Namely, reduction of a content of a low molecular weight component is not made sufficiently and is still insufficient from the viewpoint of recent market demand.
An object of the present invention is to provide a novel fluorine-containing elastomer which has few branched chains and undergoes a small change in its weight at high temperatures (reduced thermal deterioration and evaporation) and to provide a process for preparation of the elastomer.
Another object of the present invention is to provide a fluorine-containing elastomer composition which has a high vulcanization efficiency and excellent mold-processability and provides a vulcanized article which undergoes a small compression set.